Which one of the following elements is unable to form $MF_{6}^{3-}$ ion?

Which of the following is the strongest Lewis acid?

Borax is actually made of two tetrahedra and two triangular units joined together and should be written as: $Na_2[B_4O_5(OH)_4] \cdot 8H_2O$. Consider the following statements about borax:
$a.$ Each boron atom has four $B-O$ bonds
$b.$ Each boron atom has three $B-O$ bonds
$c.$ Two boron atoms have four $B-O$ bonds while other two have three $B-O$ bonds
$d.$ Each boron atom has one $-OH$ group
Select correct statement$(s)$:

When borax is dissolved in water,the product formed is

The formula of borax is

Which one of the following oxides dissolves in both hydrochloric acid and sodium hydroxide?